The first continuous distillation tower built was the “patent still” used in Britain to produce Scotch whiskey, in 1835. The patent still is to this day employed to make apple brandy in southern England. The original still, and the one I saw in England in 1992, had ordinary bubble-cap trays (except downpipes instead of downcomers were used). The major advantage of a bubble-cap tray is that the tray deck is leakproof. As shown in Fig. 4.5, the riser inside the cap is above the top of the outlet weir. This creates a mechanical seal on the tray deck, which prevents liquid weeping, regardless of the vapor flow.
Bubble-cap trays may be operated over a far wider range of vapor flows, without loss of tray efficiency. It is the author’s experience that bubble-cap trays fractionate better in commercial service than do perforated (valve or sieve) trays. Why, then, are bubble-cap trays rarely used in a modern distillation?
There really is no proper answer to this question. It is quite likely that the archaic, massively thick, bolted-up, cast-iron bubble-cap or tunnel-cap tray was the best tray ever built. However, compared to a modern valve tray, bubble-cap trays
• Were difficult to install, because of their weight.
• Have about 15 percent less capacity because when vapor escapes from the slots on the bubble cap, it is moving in a horizontal direction. The vapor flow must turn 90°. This change of direction promotes entrainment and, hence, jet flooding.
• Are more expensive to purchase.
But in the natural-gas fields, where modern design techniques have been slow to penetrate, bubble-cap trays are still widely employed, to dehydrate and sweeten natural gas in remote locations.